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That usually would indicate that either a bolt or roll pin on the side not moving has sheared off or come loose. This would be on the gear tracks underneath. When a roll pin shears it can be hard to see when it doesn't fall out. You have to watch the gear and shaft carefully to see if they move together or separately. Take a look at that as someone moves the room just a bit and let me know what you see and we can go from there.
IF you can see the gear tracks then you should see the cross shaft. If this was working when you stored it, I wouldn't think this would happen. If I know the make of your slide hardware it would help. Some have more than one motor and that might be in play here.
Just checking back to see if you were able to resolve this issue or do you still need some additional input from me?